DE102017215561A1 - FMCW radar sensor with synchronized high frequency components - Google Patents

Abstract

An FMCW radar sensor comprising a plurality of radio frequency components (10, 12) synchronized with one another by a synchronization signal (sync), at least one of which has a transmission part (16) for generating a frequency modulated transmission signal (TX), and at least two of them spatially separated high-frequency components (10, 12) each having a receiving part (20) for receiving a Radarechos (E), each receiving part (20) a mixer (22) by mixing the received signal (RX) with a portion of the transmission signal (TX) generates an intermediate frequency signal (Z1, Z2), and an evaluation unit (24, 34) are assigned and the evaluation unit (24, 34) is adapted to record the intermediate frequency signal (Z1, Z2) over a measurement period as a function of time and subjecting the time signal (S1, S2) thus obtained to a Fourier transformation, characterized in that at least one (34) of the evaluation units is adapted to Compensation of a transit time difference of the synchronization signal (sync) between the receiving parts (20), the time signal (S1) before the Fourier transform with a complex valued window function (V) to windows.

Description

The invention relates to an FMCW radar sensor having a plurality of radio frequency components which are synchronized by a synchronization signal and comprise at least two spatially separated radio frequency components, each having a transmitting part for transmitting a frequency modulated transmission signal and / or a receiving part for receiving a radar echo, each receiving part a mixer which generates an intermediate frequency signal by mixing the received signal with a portion of the transmission signal, and an evaluation unit are assigned and the evaluation unit is adapted to record the intermediate frequency signal over a measurement period as a function of time and to subject the time signal thus obtained to a Fourier transform.

State of the art

In known FMCW radar sensors, the frequency of the transmission signal is modulated in a ramp. In the receiving part is obtained by mixing the received signal with the transmission signal an intermediate frequency signal whose frequency depends on the frequency difference between the currently transmitted signal and the received signal. Due to the ramp-shaped modulation, this frequency difference depends on the transit time of the radar waves from the sensor to the object and back to the sensor. By Fourier transformation one obtains a spectrum of the intermediate frequency signal in which each located object emerges as a peak at a frequency dependent on the distance of the object. Due to the Doppler effect, however, the frequency position of the peak also depends on the relative speed of the object. In order to separate the distance-dependent and speed-dependent portions from each other, it is known to drive a plurality of frequency ramps with different pitch one after the other. Since only the distance-dependent component of the frequency is dependent on the ramp gradient, the distance and the relative speed of the object can be determined by comparison of the frequency positions obtained on the various ramps.

The fact that the measurement period over which the time signal is recorded can only have a limited length leads to the fact that the Fourier transformation generates artifacts in the form of secondary maxima which make the interpretation of the signal more difficult. It is known to largely suppress such secondary maxima by "windowing" the time signal before the Fourier transformation with a suitable window function, for example by multiplying the time signal by a likewise time-dependent window function. The window function, for example a so-called Hamming window, has primarily the effect that the abrupt transitions in the time signal are smoothed at the beginning and at the end of the measurement period and thereby the secondary maxima are mitigated.

Radar sensors of this type are already widely used as sensory components in driver assistance systems for motor vehicles. In the course of further development of the driver assistance systems in the direction of highly autonomous driving, the performance of the radar sensors is subject to increasingly greater demands. A relatively inexpensive way to meet these requirements is to have multiple components of the same type run in parallel, rather than developing new and more powerful components. This makes it possible to fall back on existing and mass-produced components, but requires that the several high-frequency components are precisely synchronized with each other.

Since the various high-frequency components must necessarily have a certain spatial distance from one another, a sufficiently precise synchronization proves difficult in view of the unavoidable signal propagation time of the synchronization signal from one component to another. Although it may be possible to avoid runtime differences by symmetrical arrangement of the blocks or by detour lines, but this requires a not inconsiderable expense and an increased space requirement on the board. This is especially true in cases where the block that generates the synchronization signal as the master is to be synchronized with the other blocks (slaves). In the master then the locally generated synchronization signal must be artificially delayed.

Disclosure of the invention

The object of the invention is to allow for a radar sensor of the type mentioned a simpler synchronization of the multiple high-frequency components.

This object is achieved in that at least one of the evaluation is designed to fen to compensate for a delay difference of the synchronization signal between the receiving parts, the time signal before the Fourier transform with a complex valued window function.

The invention exploits a property of the Fourier transformation, which consists in the fact that it can be achieved by a suitable complex-valued window function that the. By the Fourier transform obtained spectrum shifts by an adjustable amount on the frequency axis. When the transmission signal is transmitted from one device and received by another device, the signal propagation time of the synchronization signal from one device to another results in a difference in frequency when mixing the received signal with the transmission signal (which in turn is synchronous with the synchronization signal) on the spectrum of the intermediate frequency signal similar effect as a changed signal delay of the radar waves, and thus simulates a change in the object distance. Since the frequency shift of the peak in the spectrum achieved by the window function can also be interpreted as (apparent) change of the object distance (the influence of the Doppler effect at non-vanishing relative speed need not be considered here), the signal propagation time of the synchronization signal can be adjusted by a suitable frequency shift Compensate by means of the window function, without consuming measures to match the run lengths of the synchronization signal to the individual blocks are required.

Advantageous embodiments and further developments of the invention are specified in the subclaims.

In one embodiment, one of the high-frequency components is operated as a master, while one or more further high-frequency components operate as slaves. In this case, both the master and the slaves each have a transmitting part and a receiving part, so that it is possible to switch between operating modes in which the master or one of the slaves transmits the transmission signal. The radar echoes can then be received by all radio frequency components, including the component that sent the transmission signal. The synchronization error can be compensated in each case with the block that receives the radar echo but does not send itself.

The invention is also applicable to a configuration in which a single RF component receives signals transmitted from different spatially separated RF components (with signal separation in, for example, time, code or frequency division multiplexing).

In the following an embodiment will be explained in more detail with reference to the drawing.

• 1 ein Blockdiagramm der wesentlichen Komponenten eines erfindungsgemäßen Radarsensors;
• 2 ein Zeitdiagramm zur Illustration der Frequenzmodulation beim einem FMCW-Radar;
• 3 Beispiele für Zeitsignale, die in verschiedenen Hochfrequenzbausteinen des Radarsensors nach 1 empfangen werden; und
• 4 Spektren der Zeitsignale nach 3.

Show it:

• 1 a block diagram of the essential components of a radar sensor according to the invention;
• 2 a timing diagram illustrating the frequency modulation in a FMCW radar;
• 3 Examples of time signals in the various high-frequency components of the radar sensor after 1 to be received; and
• 4 Spectra of the time signals 3 ,

The in 1 shown radar sensor has two identical high-frequency components 10 . 12 on, each having an antenna array 14 , a broadcast part 16 with a local oscillator 18 , and a reception section 20 with a mixer 22 contain. The antenna arrangement 14 is used both for transmitting and for receiving radar waves and is thus a common part of the transmission part 16 and the receiving part 20 , In the example shown, the high-frequency component operates 10 as master and the high-frequency module 12 as a slave. The master generates a synchronization signal sync which is sent to the high-frequency module operating as a slave 12 (and possibly to other slaves) is transmitted. The local oscillator 18 in the high-frequency module 12 (Slave) generates a frequency-modulated transmission signal TX with a ramp-shaped modulation scheme synchronized with the synchronization signal sync, and transmits this transmission signal TX (in the example shown above the mixer 22 ) to the antenna assembly 14 so that radar waves RW be radiated. The radar echo E reflected by an object, not shown, is emitted by the antenna arrangement in the radio-frequency module 12 receive. A received signal RX gets in the mixer 22 with a portion of the transmission signal TX mixed, creating an intermediate frequency signal Z1 is generated, the an evaluation 24 is issued.

The evaluation unit 24 contains a pre-processing stage 26 with a time signal module 28 in which the intermediate frequency signal Z1 digitized and recorded over a specific measurement period as a function of time. In this way, a digital time signal S1 formed to a Fourier transform module 30 the evaluation unit 24 pass and there by Fourier transformation into a spectrum F [S1] is converted. The spectrum is in 1 also shown graphically and contains a single peak 32 whose frequency position indicates the distance of the located object (for the sake of simplicity, it should be assumed here that the relative speed of the object is zero, so that there is no Doppler shift).

The high frequency module working as master 10 In this example it is not used for sending, but only for receiving the radar echo E. The local oscillator 18 in the high-frequency module 10 generates the transmission signal TX which is frequency modulated in the same way as in the high frequency device 12 , wherein the modulation scheme is synchronized with the synchronization signal sync stored in the high frequency device 10 is generated locally. The transmission signal TX In this case, however, it does not touch the antenna arrangement 14 forwarded but only in the mixer 22 with the received signal RX mixed, so that here too an intermediate frequency signal Z2 receives, with perfect synchronization with the intermediate frequency signal Z1 should be identical.

The intermediate frequency signal Z2 arrives at a preprocessing stage, designated here at 26 ', and from the preprocessing stage 26 only differs in that the time signal module 28 a window module 36 is downstream, in which the from the intermediate frequency signal Z2 generated time signal S2 with a window function V is fenestrated. This will be a modified time signal S2_m then formed in the Fourier transform module 30 is subjected to Fourier transformation. The spectrum obtained in this way
F [S2_m] should ideally with that in the high frequency component 12 obtained spectrum F [S1] be identical and thus a peak 38 show its frequency position with that of the peak 32 matches. Under this condition, the spectra obtained in the two (or more) radio frequency components can be subjected to a combined evaluation in order to achieve greater performance of the radar sensor. For example, the two spectra can be added to improve the signal-to-noise ratio.

However, one complication arises from the fact that between the two high-frequency components 10 . 12 inevitably there is a certain spatial distance, so that the synchronization signal sync from the master to the slave has to travel a certain signal path d. Accordingly, the synchronization signal is when it is on the radio-frequency module 12 arrives to delay a signal propagation time d / c (c is the propagation velocity of the synchronization signal). This delay results in a synchronization error between the modulation patterns of the transmission signals TX in the two high-frequency components.

In 2 a (simplified) example of a modulation scheme is shown. The frequency f_r of the transmission signal is shown here as a function of time t and has a sequence of modulation ramps 40 with a ramp slope B / T, where B is the frequency sweep and T the duration of the modulation ramp is. This duration T is at the same time the duration of the measurement period over the time signal in the time signal module 28 is recorded.

In the high-frequency module 12 , which works as a transmitter here, is the beginning of each modulation ramp 40 opposite the beginning of the modulation ramp in the high-frequency component 10 delayed by the signal delay d / c. Because in both high-frequency components 10 . 12 the same received signal RX is received, however, this signal with mutually delayed transmission signals TX is the frequency of the time signal S2 in the high-frequency module 10 not determined solely by the distance of the located object and the corresponding signal propagation time of the radar waves, but it still contains an additional proportion, which is due to the fact that the modulation ramp 40 in the high-frequency module 10 already started around the time d / c earlier. The window module 36 in the evaluation unit 26 ' for the high-frequency module 10 has the purpose of compensating for this frequency offset.

multipliziert. Darin ist j die Wurzel aus (-1), pi ist die Kreiszahl, T ist die Dauer der Messperiode und zugleich die Rampendauer, b ist ein sogenannter Binversatz, der so gewählt wird, dass der Synchronisationsfehler ausgeglichen wird, und x ist ein beliebiger Wert aus dem Intervall [0, T], der eine konstante Phasenverschiebung bewirkt. Als vorteilhaft hat sich x = T/2 erwiesen.In 3 are the time signals S1 and S2 represented as functions of time t. On the vertical axis, only the real part ReA of the normalized (complex) amplitude A is indicated here. It can be seen that the frequency of the time signal S2 due to the above-described synchronization error with respect to the frequency of the time signal S1 is increased. In the window module 36 This frequency offset is reversed again, so that ideally the modified time signal S2_m with the time signal S1 matches. This is done in the window module 36 the time signal S2 , ie the time-dependent function S2 (t) with an equally time-dependent window function $$\text{V}\left(\text{t}\right)=\text{exp}\left(-\text{j}*2*\text{pi}*\left(1/\text{T}\right)*\left(\text{t}-\text{x}\right)*\text{b}\right)$$

multiplied. Where j is the root of (-1), pi is the circle number, T is the duration of the measurement period and at the same time the ramp duration, b is a so-called bin offset, which is chosen to compensate for the synchronization error, and x is an arbitrary value from the interval [0, T], which causes a constant phase shift. It has proved to be advantageous x = T / 2.

The window function V (t) is a complex-valued function whose magnitude is constant 1 and whose phase is proportional to the time t and to the bin offset b. The term "bin offset" results from the fact that the range of frequencies f on which the spectra F [S2_m] and F [S1] are divided into a plurality (for example, 512) bins each having a bin width W = c / 2B, as in FIG 4 is shown.

In 4 is also a spectrum for comparison F [S2] shown by Fourier transform of the time signal S2 would receive, so without windowing with the window function V , It can be seen that the corresponding peak in the spectrum is at a slightly higher frequency than the peak in the spectra F [S2_m] and F [S1] , coinciding with the frequency difference, which is also in 3 can be seen.

It should be noted that the bin width W has the dimension of a length while on the horizontal axis in 4 the frequency f is given as an independent variable. For the radar echo of an object with the object distance D is the frequency f at which the peak originating from the object lies, but is given by $$\text{f}=\left(\text{B}/\text{T}\right)*2\text{D}/\text{c}$$

The frequency f can thus also be used as a measure of the object distance D to be viewed as. In the 4 Frequency bins shown are therefore equivalent to distance bins with the bin width W.

The bin offset b is given by the ratio between the run length d of the synchronization signal and the bin width W, therefore $$\text{b}=\text{d}/\text{W}=\text{d}*\text{2 B}/\text{c}$$

Under these conditions, the frequency offset is between the peaks in the spectra F [S2] and F [S2_m] equivalent to an apparent change in the object distance D which is equal to the run length d. Consequently, the spectrum falls F [S2_m] in 4 essentially with the spectrum obtained in the other frequency component F [S1] together, so that the synchronization error is compensated.

In a practical embodiment, the preprocessing stage also becomes 26 that the high frequency component 12 is assigned, a window module 36 contain. The window function V In addition to the complex phase factor given in equation (1), it can also contain a real factor which serves to suppress secondary maxima. The window modules in the two preprocessing stages can then each be switched between a window with a complex phase factor and a real window without this factor, depending on whether or not a synchronization error has to be corrected. For example, it is also possible to realize an operating mode in which the master, that is to say the high-frequency component, is realized 10 , transmits and receives while the slave, so the high-frequency component 12 , exclusively receives. In that case, the complex phase factor would be at the high frequency device 10 deactivated and at the high-frequency module 12 activated.

Likewise, with an appropriate adaptation of the window function, an operating mode would also be conceivable in which the high-frequency component is 12 the master and the high-frequency module 10 forms the slave.

The synchronization signal sync could in principle also be generated directly by the transmission signal generated by the master TX be formed. When the slave transmits, it would then simply transmit the synchronization signal received from the master as a transmit signal to the antenna array 14 hand off. The slave then did not need to have its own local oscillator.

Claims (5)

FMCW radar sensor having a plurality of radio-frequency components (10, 12) which are synchronized with one another by a synchronization signal (sync) and comprise at least two spatially separate radio-frequency components (10, 12) each having a transmitting part (16) for transmitting a frequency-modulated one Transmit signal (TX) and / or a receiving part (20) for receiving a radar echo (E), wherein each receiving part (20) has a mixer (22) by mixing the received signal (RX) with a portion of the transmission signal (TX) generates an intermediate frequency signal (Z1, Z2), and an evaluation unit (24, 34) are assigned and the evaluation unit (24, 34) is adapted to the intermediate frequency signal (Z1, Z2) over a measurement period (T) as a function of time (t ) and to subject the time signal (S1, S2) thus obtained to a Fourier transformation, characterized in that at least one (34) of the evaluation units is designed to compensate for a run unlike the synchronization signal (sync) between the receiving parts (20), the time signal (S1) is fenced before the Fourier transformation with a complex-valued window function (V). Radar sensor after Claim 1 in which at least two high-frequency components (10, 12) have both a transmitting part (16) and a receiving part (20), and the associated evaluation units (24, 34) each have a window module (36) between the complex-valued window function and a pure real window function is switchable. Radar sensor after Claim 1 or 2 in which the windowing consists in a multiplication of the time signal (S2) with a time-dependent window function (V), which - optionally in addition to a real factor with time-dependent variable amount - a complex phase factor of the form $$\text{exp}\left(-\text{j}*\text{w}*\left(\text{t}-\text{x}\right)\right)$$

where t is the time w is proportional to the signal path d which the synchronization signal (sync) between the high-frequency components (10, 12), and x is any value from the interval [0, T] and T is the duration of the measurement period. Radar sensor after Claim 3 in that the transmitting part (16) is designed to modulate the frequency of the transmission signal (TX) ramp-shaped with a frequency deviation B during the duration of the measuring period T, and in which the complex phase factor is given by $$\text{exp}\left(-\text{j}*2*\text{pi}*\left(1/\text{T}\right)*\left(\text{t}-\text{x}\right)*\text{b}\right)$$

with b = d * 2B / c, where c is the propagation velocity of the synchronization signal, and d is the length of the signal path of the synchronization signal (sync) from one high frequency device to the other. Radar sensor after Claim 3 or 4 , with x = T / 2.

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